Fillers, such as carbon black are added to elastomeric compounds for a variety of reasons. They act as a low cost diluent and as reinforcing agents, giving higher modulus, higher strength and greater wear resistance. The interaction between filler and an elastomer matrix is also very important to the enhancement of desirable compound properties such as hysteresis and abrasion resistance as well as tire properties such as rolling resistance and treadwear (see FIG. 5). It is believed that when the interaction between the carbon black filler and the polymer matrix is increased that dynamic properties are improved. This is generally evidenced by lower hysteresis at elevated temperatures that would result in lower rolling resistance when the rubber is used to make automobile tires. Increased interaction between the polymer and filler can also result in cured rubber with lower heat build-up. Interaction of rubber to the filler also results in changes in other properties. When rubber and carbon black interact, the amount of bound rubber increases. This is due to some of the polymer being strongly bonded to the surface of the carbon black. This is demonstrated by dissolving the uncured rubber in a good solvent, leaving the carbon black and bound polymer as a gel. In the absence of interaction, this quantity of gel will be minimal, and when interaction increases the amount of gel will increase. The increase in the amount of bound rubber gel in the uncured compound is generally taken as evidence of increased interaction between the filler and polymer (see FIG. 1).
In the absence of interaction between filler and its elastomeric matrix, the filler forms a loosely bonded network within the matrix, which remains after curing. When the dynamic storage modulus, designated G′, is measured in the cured rubber sample, the filler network acts to increase the modulus at low strain. As the applied strain on the rubber sample is increased, the bonds that form this filler network are broken, and it no longer contributes to the modulus. Thus, in the presence of small or low interaction between filler and the elastomer matrix, the dynamic storage modulus G′, will diminish as the applied strain is increased. This is known as the Payne Effect. As the filler to polymer matrix interaction increases, the filler—filler network should be decreased in the final cured elastomer. Thus when applied strain is increased as the dynamic measurement is made, the storage modulus, G′, does not decrease as rapidly with an increase in the strain. The diminution of the Payne Effect is also taken as evidence that increased filler—polymer interaction has taken place (see FIG. 4). Another way to measure this is by the % G′ Retained which is simply the ratio of low strain/high strain, where higher is better.
Similarly, when the modulus of a cured sample is measured in simple extension, the modulus will increase as the strain increases. When a sample that has increased filler interaction to the matrix is compared to a control, the ratio of the modulus at high strain to the modulus at low strain will be higher. Thus, an increase in the ratio of the modulus at 300% extension to the modulus at 5% extension, (M300/M5), may be taken as evidence that additional interaction has taken place. Thus this ratio, known as the reinforcement factor, is a measure of increased polymer-filler interaction (see FIG. 2).
In the past some chemicals have been added to rubber to improve the interaction of carbon black with the rubber matrix. For example N-methyl-N, 4-dinitrosoaniline was used, but it was discontinued due to its toxicity. Benzofurazan oxides have also been reported to be effective coupling agents, but upon curing they evolve an undesirable odor. Xanthogen polysulfides have been known to the rubber industry for some time. They have been used as a source of sulfur in vulcanization or as ultra accelerators for sulfur vulcanization. For example, Stevenson U.S. Pat. No. 4,695,609 points out that “U.S. Pat. No. 1,634,924, U.S. Pat. No. 2,374,385 and U.S. Pat. No. 2,453,689 each disclose the use of dihydrocarbyl xanthogen polysulfides as accelerators in rubber compositions. It is stated in U.S. Pat. No. 1,634,924 (and proved by the given Examples) that the additional presence of an amine “of the aniline type” in the composition is advantageous. It is also stated, although there is no evidence, that no free sulfur need be added. In U.S. Pat. No. 2,374,385, a thiazole or other N-containing compound is invariably used as an accelerator; under the acid conditions, thiazole tautomerism can give nitrosatable secondary amines. In U.S. Pat. No. 2,453,689, ‘base stock’ used for those vulcanisates having the best properties includes a sulphenamide or urea, and alternative N-containing accelerators are suggested. The highest recorded tensile strength is 2700 lb/in2 (18600 kPa). Example VIII of U.S. Pat. No. 1,634,924 discloses curing a mixture comprising 100 parts smoked sheet (natural rubber), 5 parts ZnO, 5 parts sulphur and 1/25 parts diisoamyl xanthogen tetrasulphide, at about 116 C. This is the only instance given in which no amine is used, and the state of cure is very poor by comparison with the products of the other Examples in which dibenzyl amine, ethyl aniline or aniline is present. The amount of sulphur is such that it will almost certainly bloom. In one reported case in U.S. Pat. No. 2,453,689, a rubber stock comprising solely 100 parts Buna S (synthetic rubber), 55 parts carbon black and 5 parts diethyl xanthogen tetrasulphide is vulcanised at about 120 C. It should be noted that neither zinc oxide nor sulphur is present. The results are said to show that ‘xanthic sulfides are very active vulcanizing agents even in the absence of auxiliary agents such as accelerators and activators’, but the product's tensile strength is relatively low, i.e. 1280 lb/in2 (8825 kPa). In neither of the given specific instances from the prior art is the product likely to be of practical utility. A tensile strength of at least 10,000 and very often at least 20,000 kPa is desirable. Perhaps for this reason, among others, xanthogen polysulphides as described in the given prior art appear not to have been used on any commercial scale, over the last 50 years.”
Stevenson then goes on to disclose vulcanizable compositions comprising rubber, a dihydrocarbon xanthogen polysulfide and an xanthate (column 3, lines 10 to 16), wherein the xanthogen polysulfide is a curing agent (column 4, lines 50 to 52). Stevenson also points out in the Example (column 5, lines 65 to 68, column 6, lines 46 to 51, etc.) that the products of his invention are comparable to prior art products except that he has minimized the presence of environmentally undesirable chemicals. Accordingly, Stevenson does not appreciate that XDS can be used to improve the properties of rubber vulcanization.
German Democratic Republic Specification 223720 A1 discloses a process for modifying elastomers or elastomer mixtures, characterized in that diorgano xanthogen disulfides are incorporated in the elastomers or elastomer mixtures at 30° C. to 220° C. followed by further processing and vulcanization at 100 to 250° C. This specification does not disclose reacting the XDS with filler, such as carbon black, simultaneously with or before reaction with elastomer. Our studies have shown that it is essential to react carbon black with XDS either before or simultaneously with rubber. If rubber is reacted with XDS in the absence of rubber, Mooney Scorch Time is undesirably less, Mooney riscosity is undesirably higher, hysteresis at 5 to 14% strain is undersirably higher.
In one aspect, this invention is a process of producing unvulcanized rubber useful for producing vulcanized rubber with improved hysteresis, which comprises mixing a mixing a composition comprising unvulcanized rubber, carbon black and xanthogen polysulfide at an elevated temperature in a non-productive stage.
In a second aspect, this invention is a composition suitable for producing unvulcanized rubber useful for producing vulcanized rubber with improved hysteresis, which comprises unvulcanized rubber, a filler comprising carbon black and xanthogen polysulfide.
In a third aspect, this invention is a process of producing vulcanized rubber with improved hysteresis, which comprises (1) mixing unvulcanized rubber, a filler comprising carbon black and xanthogen polysulfide without other curative ingredients in a non-productive mixing step, then (2) adding the remaining curative ingredients in subsequent mixing steps and vulcanizing the rubber.
In a fourth aspect, this invention is a composition comprising carbon black and xanthogen polysulfide.
In a fifth aspect, this invention is a composition comprising carbon black coated with xanthogen polysulfide.